Conventional wireless telephones use what is commonly referred to as encoder/decoder technology. The transmit path of a wireless telephone encodes an audio signal picked up by a microphone onboard the wireless telephone. The encoded audio signal is then transmitted to another telephone. However, prior to being encoded, the audio signal is typically processed to reduce the amount of ambient noise that will be transmitted and eventually heard by a user of the far-end telephone.
In order to process the audio signal, the audio signal that is picked up by the microphone is first converted from an analog signal to a digital signal by using an analog-to-digital converter. Once converted to a digital form, the audio signal is further processed using a noise suppression algorithm. Many noise suppression algorithms use a discrete Fourier transform (DFT) to approximate the magnitude of the audio signal at various frequencies. The noise suppression algorithms use the results of the DFT to determine the signal-to-noise ratio (SNR) for the various frequencies which make up the audio signal. Once the SNR has been calculated, the signal magnitude in each of the frequency bins is attenuated in accordance with the SNR to remove the ambient noise from the audio signal. After the audio signal has been cleaned or had the noise filtered out, it is transmitted to a far-end telephone where it is processed and presented to a user of the far-end telephone.
As the length of the telephone call increases, the noise suppression algorithm is able to more accurately predict the level of noise present in the audio signal picked up by the microphone as the noise estimates are based on a larger sample of data. Accordingly, when the telephone call is initially placed, there is typically a delay before the noise is accurately filtered out by the noise suppression algorithm since the algorithm has little or no data with which it determines the SNR. Similarly, when a cellular telephone switches from one network to another, the switching operation may require the noise suppression algorithm to restart because of the different configurations of the first and second networks. Accordingly, the restarting of the noise suppression algorithm may result in an increase in the amount of ambient noise that is heard by the far-end user as the noise suppression algorithm restarts and begins gathering data. This increase in noise may occur even though the ambient noise in the environment in which the near-end user is speaking remains relatively constant.
Thus, an improved method of noise suppression for wireless telephones switching from one network to another is desired.